Conventional high pressure/high temperature (HP/HT) apparatuses are known in the art, for use in the preparation of crystals such as diamond, cubic boron nitride (CBN), hexagonal boron nitride (HBN), semiconductor crystals such as gallium nitride (GaN) crystals and the like, as disclosed in U.S. patent application Publication No. 2003/0140845.
HP/HT apparatuses comprising a vessel or cell assembly may be of the belt-type, die-type, belt/dye type, or a zero-stroke type, are described, for example, in U.S. Pat. Nos.; 2,941,241; 4,523,478; 6,375,446; the disclosures of which are incorporated herein by reference. It is not uncommon for these types of HP apparatuses to contain up to 800,000–1,000,000 psi and to go up in temperatures of 2,000° C. and more.
The growth of large single crystals in such HP/HT apparatuses normally is carried out with a temperature gradient. The solubility of the substance to be crystallized is greater at one end of the cell than the other, allowing for dissolution of “source” or “nutrient” material at one end, transport (e.g., by diffusion) to the other end, and epitaxial deposition onto one or more growing crystals. The supersaturation and growth rate are controlled by the magnitude of the temperature gradient. The temperature gradient normally is achieved by means that do not allow for adjustment during the run, e.g., the growth region is asymmetrically placed within the cell so that one end is hotter than the other.
In the cell of a typical HP/HT apparatus in the prior art, a heating element makes electrical contact with two points of the apparatus, for example, with the top and the bottom anvils, and the cell is heated by passing electrical current through the heating element. External components of the pressure vessel are cooled, typically by forced convection with a coolant such as water or a water/anti-freeze solution. The mean cell temperature increases as an increasing electrical current is passed through the heating element. However, the hot end of the cell is hotter than the cold end at all times during the run, typically by an amount that is approximately proportional to the mean temperature increase of the cell relative to its initial value. Because the cell contains only one independent heating circuit and cooling is provided at a fixed temperature, it is not possible to independently control the temperatures of two different regions of the sample or locations within the cell.
For growing crystals or processing materials at modest pressures, U.S. Pat. No. 6,273,948 by Porowski et al. discloses a three-zone/independently-heated furnace for crystal growth in a gas-pressure vessel at pressures up to 20 kbar. However, gas-pressure vessels have significant limitations for crystal growth and material processing under extreme conditions. First, the volumes and pressures achievable with gas-pressure vessels are limited. Large-volume gas-pressure vessels are normally limited to a maximum working pressure of about 2 kbar. Small-volume internally-heated pressure vessels are commercially available at pressures up to about 10 kbar. Second, gas-pressure vessels are hazardous, particularly when working at pressures above 5 kbar.
By contrast, HP/HT apparatuses that utilize a solid or a liquid as a pressure medium, which are significantly less hazardous than gas-pressure vessels, can access considerably higher pressures, to 100 kbar and above. However, there is still a need for in the art for independently controlling the temperature of at least two different regions in these HP/HT apparatuses for growing crystals or processing materials with a solid or liquid pressure medium at pressures between about 2 kbar and about 100 kbar, or more particularly at pressures between about 5 kbar and about 80 kbar.
The applicants have built a surprisingly simple and novel HP/HT apparatus that allows the operator to independently control both the mean temperature in the cell and the temperature gradient across the cell. In one embodiment of the invention, improved crystal growth is also obtained in the novel apparatus of the invention.